EFFECTS OF A GROWTH IMPLANT AND AGE OF DAM ON GROWTH AND FERTILITY INHEIFER CALVES

A field trial was conducted to determine the effects of a growth impla nt on growth, pregnancy rate, calving difficulty and calf weaning weig ht in replacement beef heifers. Heifers were assigned equally to eithe r a control (n = 43; no implant), Ralgro (n = 43) or Synovex-C (n = 42 ) treatment group based on age of dam and age and weight at the start of the trial (implantation at 45-90 d of age). Neither Ralgro nor Syno vex-C enhanced heifer weight or average daily gain (ADG) from the star t of the trial to weaning (approximately 7 mo of age), breeding, or pr egnancy determination. At breeding, Synovex-C implanted heifers had gr eater pelvic area than controls while Ralgro implanted heifers were in termediate (control = 149.7, Ralgro = 154.7 and Synovex-C = 157.4 pool ed SE of 2.2 cm2; P = 0.05). Pelvic dimensions were similar among trea tments at pregnancy determination and there were no treatment effects on calving difficulty. Age of dam affected heifer weight (P = 0.0005) and ADG (P = 0.001) to weaning, with heifers from mature cows weighing more at weaning and having a higher ADG from the start of the study t o weaning than heifers from either first- or second-calf cows. Post-we aning performance of heifers from first-calf cows was similar to heife rs from mature cows while heifers from second-calf cows were lighter a t breeding (P = 0.04) and had a lower ADG from weaning to breeding (P = 0.04). Age of dam did not influence pregnancy rate (P = 0.10), but m ore heifers from second-calf cows calved late in the calving season (g reater-than-or-equal-to 64 d) compared with heifers from either first- calf or mature cows (P = 0.04). It appears from this field trial that use of a growth implant between 45 and 90 d of age neither enhanced gr owth nor impaired fertility in beef heifers bred as yearlings. It also appears that second-calf 3-yr-olds may need to be managed separately from the mature cow herd in order to optimize replacement heifer devel opment.